Retroreflective articles are well known and are used in a variety of applications. The articles have the ability to return substantial quantities of incident light--which light would otherwise be reflected elsewhere--back towards the light source. This unique ability has promoted the widespread use of retroreflective articles for a variety of applications, including use on highway signs to display information to motorists at nightime.
In U.S. Pat. No. 3,190,178 McKenzie established a state of the art for retroreflective sheetings by providing cellular retroreflective optical elements in a plurality of hermetically sealed cells. The hermetically sealed cells were formed by heat sealing a bead bond to an acrylic cover film via an emboss roll.
Early bead bond layers were typically composed of a high molecular weight linear thermoplastic acrylate and a pigment. In U.S. Pat. No. 4,025,159, McGrath improved upon the durability of the enclosed lens construction by heat sealing the bead bond to the cover film and bonding the two layers together through curing. Miyata, in U.S. Pat. No. 4,653,854, attached pendent hydroxyl groups to the backbone of the acrylate polymers used for the bead bond. Incorporation of polyisocyanates into the formulation allowed the bead bond to be crosslinked through the formation of urethane linkages.
While these developments helped address issues related to product durability and manufacturing, the processes generally required large amounts of solvent for coating operations. The solvent, of course, must be handled in an environmentally sound manner, and therefore it can cause an increase in processing costs and be an overall drawback in the manufacture of retroreflective sheeting.
Efforts to implement solventless bead bond technology into the construction of cellular retroreflective sheeting generally consisted of dissolving a high molecular weight acrylate polymer in one or more reactive diluents and then coating the material warm. This technique, however, also has certain drawbacks, such as a requirement to chill the product to low temperatures for bead stripping. Belisle et al., in U.S. Pat. No. 4,721,649, reported using a solventless, thermoformable two-component urethane as a polymeric bead bond layer for "embedded-lens" retroreflective sheeting. Bailey et al. (U.S. Pat. Nos. 4,897,136 and 5,064,272) reported using a thermoplastic urethane to develop a solventless bead bond for flexible cellular retroreflective sheeting.